1. Field of the Invention
The present invention relates to a carrier for developing a latent electrostatic image, which is suitably used for electrophotography, electrostatic recording, electrostatic printing and the like and a method for producing the carrier, a developer using the carrier, and an image forming method using the developer.
2. Description of the Related Art
In a dry developing system used for electrophotography, a visible image is formed by making a toner frictioned with a charging member electrostatically adhered on a latent electrostatic image. Such dry development systems include a one-component development system using toner as the main component and a two-component development system using glass beads, magnetic carriers, or carriers prepared by coating the surfaces of such materials with a resin or the like, in combination with toner.
In a developer used in such a two-component development system, minute toner particles are held on relatively large core material surfaces by an electric force produced by friction therebetween. When this developer is made close to a latent electrostatic image owing to an electric field formed by the latent electrostatic image, the toner overcomes the binding force with the core material, and transferred on the latent electrostatic image to develop this image. Then, the developer is repeatedly used while being replenished with the toner consumed by development.
Therefore, during use over an extended period of time, the core material must always frictionally charge the toner with a desired polarity and to a sufficient amount of charge. However, due to inter-particle collision, mechanical stirring of particles within a development device, or heat generation caused by these phenomena, a phenomenon called “spent toner” occurs where the toner is fusion-bonded to the surface of the core material, and thereby charge characteristics of the core deteriorates with operating time. As a result, since background smearing of an image and toner scattering occur, it becomes necessary to replace the entire developer.
For preventing such a spent phenomenon, many proposals have been made for extending the lifespan of carriers by coating the surface of the core material with a resin having a low surface energy, for example, a fluorine resin, a silicone resin, or the like. The proposed carriers include, for example, a carrier coated with a room-temperature curable silicone resin and a positive charge nitrogen resin (see Japanese Patent Application Laid-Open (JP-A) No. 55-127569), a carrier coated with a coating material containing at least one type of modified silicone resin (see JP-A No. 55-157751), a carrier having a coating layer containing a room-temperature curable silicone resin and a styrene-acryl resin (see JP-A No. 56-140358), a carrier for which the surface of a core particle is coated with two or more layers of silicone resin, with no adhesion provided between the adjacent layers (see JP-A No. 57-96355), a carrier for which a multilayered silicone resin is applied to the surface of a core particle (see JP-A No. 57-96356), a carrier whose surface is coated with a silicone resin containing silicon carbide (see JP-A No. 58-207054), a positively charged carrier coated with a material having a critical surface tension of 20 dyn/cm or less (see JP-A No. 61-110161), and a developer composed of carriers coated with a coating material containing fluorinated alkyl acrylate and a toner containing a chromium-containing azo dye (see JP-A No. 62-273576).
Recently, there has been a trend to reduce the particle diameter of toner for realizing a high image quality, and as a result, toner spent to the carrier easily occurs. In addition, by a conventional spray coating, it is difficult to uniformly moisten the carrier surface with a coating material, and therefore, it is difficult to produce a carrier having adhesion between the coating film and core material and a uniform film thickness and film quality. Furthermore, in the case of a full-color toner, since a low-softening-point resin is used in order to obtain a sufficient color tone, in comparison with that of a black toner, the spent amount to the carrier is large, the amount of toner charge is lowered, and toner scattering and background smear occur. Thus, a full-color electrophotographic system has a problem that the image density of, in particular, a highlight is easily changed and a high image quality cannot be maintained when the amount of toner charge is lowered.
When carrier having higher electric resistance is employed for developer, generally, the resulting image is provided sharply with a so-called edge effect such that image density is very light at central portion and dense only at edge portion in a copy image of larger area. In cases where images are characters or thin limes, the edge effect may result in clear images, but there arises such a drawback that reproducibility of images is very poor in cases of halftone images. Therefore, particularly in case where a full-color toner is used, the electric property of carrier is necessary to be appropriately controlled so as to form a halftone image having high quality and a sufficient color tone.
For improved carrier durability and formation of high quality image, it has also been proposed to provide a coating layer of resin matrix composed of low surface energy substance, in which fine particles and a conductivity-imparting material are dispersed, to thereby control spent resistance, film strength, and electrical characteristics (see JP-A Nos. 09-319161, 09-269614, and 10-186731). However, in these proposals, since a dispersion prepared by adding fine particles to an organic solvent is applied by high-temperature spray coating, there is a problem that it is difficult to unify the amount of charge due to aggregation of the fine particles.
It has been also proposed that as a conductivity-imparting material, a coating layer, in which a conductive polymer substance is dispersed in a resin, is disposed on a surface of a core material so as to control electric resistance of the carrier surface (Japanese Patent (JP-B) No. 2626754). However, it is difficult for the conductive polymer to be uniformly dispersed in the coating layer because of poor compatibility between resins, and stable resistance cannot be obtained.
Conventional spray coating techniques also suffer from operational problems, such as regulation of volatile organic compounds (VOC) resulting from an organic solvent used for formation of a coating layer on the surface of the core material, generation of liquid waste, and requirement of energy for drying.
In order to overcome these problems, for example, as methods for producing carrier that involve an organic solvent-free dry powder process, production methods that involve use of a supercritical fluid have been proposed (see U.S. Pat. No. 5,514,512). However, this proposal discloses application of a coating resin, which has been polymerized in a supercritical fluid, by melting it in a heated state, and the coating resin is an acrylic resin; thus the aforementioned toner spent problem has not yet been solved.
Furthermore, in the method for producing carrier using a supercritical fluid (see JP-A No. 2006-106208), a silicone resin is used for the coating material; however, its solubility to the supercritical fluid is insufficient, and a carrier is produced through a process in which a plasticized and microdispersed silicone resin is sprayed simultaneously with a core material for coating of the core material surface. For this reason, it results in non-uniform thickness of the coating layer, and therefore, it results in failure to obtain a coating layer thickness sufficient to ensure carrier durability. Moreover, the method for producing the carrier includes a step of dispersing a conductivity-imparting material in a resin matrix. However, since the conductivity-imparting material is dispersed by kneading, it is not sufficiently finely dispersed. Therefore, the problem of non-uniform composition caused by aggregation of the fine particles has not been solved.
Accordingly, there has not yet been provided a carrier that has a coating layer having a uniform and appropriate thickness on the surface of a core material and has a high adhesion property between the core material and the coating layer and sufficiently satisfiable related techniques thereof. Further, from the perspective of environmental burden and resource-saving, the current situation is that conventional methods of producing a carrier and conventional carriers have various problems to be solved.